1. Field of the Invention
The subject invention relates generally to communication systems and more particularly to a method and apparatus for improving communication between a cellular phone and a phone on the PSTN network whenever a digital speech compression algorithm is followed by an compander conversion, which is typical.
2. Description of Related Art
In the prior art, situations arise where a digital cellular phone (e.g., GSM, PCS- 1800, IS-54) is connected to a telephone on the public switched telephone network (PSTN). Such cellular systems typically employ a speech coder followed by a compander, such as a xcexc-Law or A-Law conversion, in order to interface to the PSTN network. Due to the xe2x80x9cpoorxe2x80x9d quantization characteristics of A-Law and to a lesser extent of the xcexc-Law conversion, at very low levels (hardly audible), the output of the speech coder are transformed into an annoying audible noise after the A/xcexc-Law conversion at the receiving PSTN phones. The problem becomes worse as the bit-rate of the speech coding algorithm decreases and is most noticeable if a level adjustment (increase) takes place after the A/xcexc-Law decoding.
It has been discovered that annoying audible noise during speech intervals can be eliminated by adding a fixed number in the digital domain to the output of the speech coder. In this manner, signal samples shifting around zero are moved away from the area of xe2x80x9cpoorxe2x80x9d quantization of the compander. The invention would typically be a part of the speech decoder algorithm, as a post operation. However, it can be used as successfully, as a stand alone block between any speech decoding algorithm and the A-Law or xcexc-Law conversion (compander). Adding a fixed number to the output of the speech decoder is inaudible, and does not degrade speech quality in any way. The technique eliminates the problem for any bit-rate. The effect is dramatic and will solve the noise problem for most existing standards. Alternatively, the constant added to the output of the speech coder is confined to a small value added to the speech coder output so that during the silence period between speech, when the output of the coder falls slightly below zero or slightly above zero, the constant value moves the entire speech coder output during the silence period slightly above zero or slightly below zero.